Technically, LGPL option allows using in proprietary software. Practically, static linking is often better e.g. makes executable much smaller. And even with dynamic linking, see this note from https://blog.qt.io/blog/2009/11/30/qt-making-the-right-licen...

> If you are shipping an executable, you must include any data and utility programs needed for reproducing the executable (i.e. must provide components needed to recompile with a new version of the library) unless such data and utility programs are normally distributed with the major components of the operating system on which the executable runs

For developers of proprietary software, flutter’s BSD 3 clause license is way easier to comply.